Structural Lithium Ion Battery Electrolytes

Abstract: A major challenge in the electrification of vehicles in the transport industry is that batteries are heavy, which reduces their effectiveness in mobile applications. A solution to this is structural batteries, which are batteries that can carry mechanical load while simultaneously storing energy. This can potentially lead to large weight savings on a systems level, since they may allow replacement of load bearing structures with structural batteries. Carbon fibers are suitable for structural batteries because they have superb mechanical properties and readily intercalate lithium ions, i.e. they can be used as electrodes in a lithium ion battery. However, to utilize carbon fibers in structural batteries, a polymer (matrix) is needed to form a composite battery. The polymer is required to have high modulus and high ion transport properties, which are inversely related, to function as an electrolyte. This thesis focuses on the development and characterization of such polymer electrolytes.The first study was performed on a homogenous polymer electrolyte based on plasticized polyethylene glycol-methacrylate. The influence of crosslink density, salt concentration and plasticizer concentration on the mechanical and electrochemical properties were investigated. Increases in both ionic conductivity and storage modulus were obtained when, compared to non-plasticized systems. However, at high storage modulus (E’>500 MPa) the ionic conductivity (?<10-7 S cm-1) is far from good enough for the realization of structural batteries.In a second study, phase separated systems were therefore investigated. Polymerization induced phase separation (PIPS) via UV-curing was utilized to the produce structural battery electrolytes (SBE), consisting of liquid electrolyte and a stiff vinyl ester thermoset. The effect of monomer structure and volume fraction of liquid electrolyte on the morphology, electrochemical and mechanical properties were investigated. High storage modulus (750 MPa) in combination with high ionic conductivity (1.5 x 10-4 S cm-1) were obtained at ambient temperature. A SBE carbon fiber lamina half-cell was prepared via vacuum infusion and electrochemically cycled vs lithium metal. The results showed that both ion transport and load transfer was enabled through the SBE matrix.In the third study the mechanical and electrochemical properties of the SBE-carbon fiber lamina were investigated and the multifunctional performance was evaluated. A new formulation of SBE, with a small addition of thiol monomer, were prepared with improved electrochemical and mechanical properties. The mechanical properties of the SBE carbon fiber lamina did not deteriorate after electrochemical cycling. The capacity of the SBE carbon fiber lamina half-cell was 232 ± 26 mAh g-1, at a C/20 charge rate. Furthermore, the lamina displayed multifunctional performance, compared to the monofunctional properties of its constituents.In the final study, a new curing method was investigated, since UV-curing cannot be used to prepare full-cell carbon fiber composite structural batteries. Thermal curing was investigated to prepare the SBE. The PIPS was not adversely affected by the change in curing method, and the length scale of the phase separation in the SBE was slightly larger compared to UV-cured SBEs. The thermally cured SBEs exhibited improved thermomechanical properties without a reduction in the electrochemical properties. Thermal curing did not affect the electrochemical properties of the SBE carbon fiber lamina, however the type of carbon fiber utilized was found to negatively affect the cycling performance.